In recent years, printing with inkjet printers that necessitates no printing plate has been spread to produce a limited number of copies of a recorded material, such as a local ad and a large poster. Inks for use in inkjet printers generally include aqueous type inks, solvent type inks, and ultraviolet-curable type inks. Aqueous type inkjet inks are low in water resistance when printed on standard paper, and can easily cause blurring. When printed on a non-water-absorbing recording medium such as a plastic, aqueous type inkjet inks are poor in fixing ink droplets and thus a failure in image formation is apt to occur. Aqueous type inkjet inks also have the disadvantage that since the drying process for the solvent is very slow, recorded materials immediately after printing have to be dried without being piled up or superposed.
On the other hand, solvent type inks are suitable for printing on a non-water-absorbing recording medium such as a plastic, but have the disadvantages that since the solvent on the medium has to be dried, it takes a certain period of time to perform drying, and that an air-exhausting system or a solvent-collecting equipment for the evaporation of organic solvents also have to be provided.
To solve these problems, inkjet inks that harden and deposit not by volatization of ink solvent but by irradiation of radiation ray were developed. For example, JP-A-2003-221528 (“JP-A” means unexamined published Japanese patent application), JP-A-2003-221532, and JP-A-2003-221530 disclose inks containing a monomer having a polymerizable group and an oil-soluble dye. Alternatively, JP-A-2001-222105 discloses a photopolymerizable composition containing a photopolymerizable compound and a photopolymerization initiator. Since these inks are of a radical polymerization type, however, the polymerization can be inhibited by oxygen, and thus it is apt to cause a failure in curing in the air, on which an improvement has been demanded.
Under the circumstances, there are disclosed inks using cationic polymerization that can be free from oxygen inhibition for polymerization. Known cationically polymerizable monomers that are generally used for cationic polymerization-type ultraviolet-curable type ink, include oxirane ring-containing epoxy compounds, oxetane compounds, and vinyl ether compounds. Specifically, it is known that the polymerization rate can be significantly increased, by using a combination of an epoxy compound and an oxetane compound (Japanese Patent No. 2679586). In particular, oxetane compounds have good heat resistance, adhesive properties, and chemical resistance, and thus are useful for use in combination with reactivity-enhancing epoxy compounds. Concerning cationically polymerizable ink, for example, JP-A-2000-169552, JP-A-2001-220526, and JP-A-2002-317139 disclose active energy ray-curable compositions containing oxirane or oxetane. However, these active energy ray-curable compositions have insufficient curing property, and thus the resultant cured products may be brittle and insufficient in adhesion to a recording medium. Further, JP-A-2002-317139 discloses an active energy ray-curable composition containing oxirane, oxetane, or vinyl ether. Although the active energy ray-curable composition has relatively good curing property, the resultant cured product may be brittle and insufficient in the adhesion, and the active energy ray-curable composition generates a bad smell, and thus the development of highly sensitive materials without using vinyl ether has been demanded. The ink compositions as disclosed in these patents documents have a problem in which since their curing property is not sufficient, a high-illumination-intensity ultraviolet lamp is necessary, to make the printing apparatus large in size and complicated.